Agonistic behavior of the crayfish Euastacus armatus and Cherax destructor

The intra- and interspecific agonistic behavior of Euastacus armatus and Cherax destructor from northeastern Victoria were examined. While the agonistic patterns of E. armatus appeared similar to those shown by most crayfish, individuals of C. destructor execute an unusual, highly stylized cheliped “punch” behavior during strong interactions, along with the other behaviors seen in many species. Juvenile C. destructor exhibited gregarious behavior, tending to co-occupy burrows and being physically near each other. Tests showed that the white chelipeds which are characteristic of mature E. armatus affect the outcome of aggressive interactions. When individuals had their chelipeds whitened, they won agonistic interactions more often. This result held for both intraspecific pairings and size-matched pairings with individuals of C. destructor. Individuals of C. destructor won the majority of size-matched pairings with non-whitened individuals of E. armatus.     

The evolution of Queensland spiny mountain crayfish of the genus Euastacus. I. Testing vicariance and dispersal with interspecific mitochondrial DNA

The upland mesic rainforests of eastern Australia have been described as a "mesothermal archipelago" where a chain of cool mountain "islands" arise from a warm "sea" of tropical and subtropical lowlands. An endemic freshwater crayfish belonging to the genus Euastacus is found on each of these mountain "islands." The Euastacus are particularly suitable for the study of evolution because each mountain harbors a unique species, there are many taxa present providing replication within the group and, most importantly, their distribution is linear, extending along a south-north axis. This group could have evolved by "simultaneous vicariance" where there was one vicariant separation event of a widespread ancestor, or by "south to north stepping stone dispersal" where there were long distance dispersal events from neighboring mountain islands, starting in the south and proceeding north in a dispersal-colonization wave. We used pairwise genetic distances between nearest geographic neighbors as a novel way to test the two hypotheses. If diversification was due to "south to north stepping stone dispersal," then pairwise genetic distances between nearest geographic neighbors should decrease progressively the farther north the taxon pairs are found, reflecting the decreasing periods of isolation. In this case there should be a negative correlation between the south to north rank order of nearest neighbors and pairwise genetic distances. A Spearman's correlation on 16S mtDNA pairwise genetic distances and geographic rank order was not significant, indicating there was no support for the south to north stepping stone dispersal hypothesis. If simultaneous vicariance was responsible for diversification then all nearest geographic neighbor taxon pairs should have similar genetic distances and, therefore, the variance in nearest neighbor distances should be zero, or close to it. To test if the observed variance was tending towards zero we developed a randomization test where nearest neighbor taxon pairs were assigned random genetic distances and the variances calculated. The observed variance lay in the < 0.05 range of the simulated variances, providing support for the simultaneous vicariance hypothesis. The data also suggest there was simultaneous vicariance of at least two ancestral Queensland lineages. The timing of this vicariant event was probably in the Pliocene, which is consistent with the divergence times reported for other Australian mesic rainforest restricted taxa.     

Collaborating with recreational fishers to inform fisheries management: Estimating population abundance for an iconic freshwater crayfish

Can the abundance of fish populations be effectively determined by the collection of scientific research with support from recreational fishers? Collecting and analysing fishing data from recreational fishers to aid management are not new; however, engaging fishers in a scientific survey design to produce specific population estimates is rarely undertaken. We engaged recreational fishers to assist with field sampling to provide an estimate Murray Crayfish (Euastacus armatus von Martens, 1866) abundance at three sites on the Edward River which were recently impacted by an extreme blackwater disturbance. Employing mark‐resight models, fishers undertook crayfish surveys and produced research data which estimated adult population sizes of Murray Crayfish in the studied reaches ranging between 94.27 ± 24.72 individuals (Below Stevens) and 450.01 ± 175.30 individuals (Twin Rivers). Both the effective undertaking of the mark‐resight designs in collaboration with fishers and acquiring population abundance estimates for Murray Crayfish in a river reach are concepts which have not previously been published and are important attributes for the management of aquatic species.     

Trophic positions of omnivores are not always flexible: Evidence from four species of freshwater crayfish

Omnivores are generally believed to be flexible in their diet and trophic position: seasonal, ontogenetic and site‐based differences in trophic position have been observed. We compared consumed and assimilated diet among four species within a group of omnivorous freshwater crayfish, to determine whether species that occur together at a site occupy different trophic positions. Diets of Geocharax falcata, Gramastacus insolitus, Cherax destructor and Euastacus bispinosus (Decapoda: Parastacidae) were compared using stable isotopes (δ¹³C and δ¹⁵N) and gut content analysis across nine sites that varied in their species composition. Gramastacus insolitus consumed mainly plant material across all sites. Geocharax falcata consumed either plants or animals or both at different sites. Its trophic level was consistently similar to G. insolitus, despite differences in gut contents and source for dietary carbon. Cherax destructor consumed animals and had a relatively stable trophic position among sites. Relative trophic position of these three species was consistent across sites and regardless of food consumed, they were positioned as omnivores at a lower trophic level than predators but higher than primary producers and herbivores. Euastacus bispinosus occupied a higher trophic level than other invertebrate species but δ¹³C levels did not differ among sites. Cherax destructor and G. falcata may show flexibility in food sources and in the assimilation of food that determines their trophic position relative to other crayfish species. In contrast, G. insolitus and E. bispinosus are likely to show both a more fixed diet and less flexible trophic position. Therefore, not all omnivores show the flexible diet and trophic position generally reported in the literature. Some species of omnivorous crayfish may maintain a relatively constant trophic position across sites, seasons or changes in food availability regardless of whether their consumed diet alters or not.     

Responses to conspecific and heterospecific disturbance odours in the Australian crayfish Euastacus armatus and Cherax destructor

The responses of individuals of Cherax destructor and Euastacus armatus to water containing stressed or unstressed crayfish were tested in the laboratory. Individuals of E. armatus showed only an increase in locomotion when stressed crayfish water was introduced. Individuals of C. destructor showed a decrease in the time spent in a lowered posture, an increase in the time spent in the raised posture and in grooming behaviour when stressed crayfish water was introduced. Individuals of C. destructor responded more strongly to heterospecific animals (Cherax rotundus and E. armatus) than to conspecifics. The levels of responses to the two heterospecific species were similar.